The present invention relates to a method and apparatus for hiding data in an audio or video signal, and more particularly to a scheme for providing an imperceptible auxiliary data signal in a compressed digital audio or video data stream being communicated over existing communication channels. The scheme is characterized as post-compression hidden data transport (PC-HDT) for video. A corresponding method and apparatus for recovering the hidden data from the audio or video signal are also disclosed, in addition to a digital data storage medium.
Generally, the capacity of a transmission channel to carry additional information is limited by the bandwidth of the channel. Since the bandwidth of communication channels is limited by realities of the electromagnetic spectrum, and, for wireless channels, may be regulated by law, techniques have been developed for increasing the amount of information that can be carried within a channel of a given bandwidth. For example, techniques for compressing digital data to squeeze more data within a given bandwidth or data storage space are well known.
Compression refers to reducing the number of data bits needed to encode a source signal so that the source signal may be recovered by a decoder with acceptable fidelity. For example, it is known that the human eye is relatively less sensitive to amplitude variations in high-frequency components of a video signal. Similarly, the human ear is relatively less sensitive to amplitude variations in high-frequency components of an audio signal. Accordingly, audio and video data can be encoded using frequency transform techniques which allocate fewer bits to high-frequency components. In this manner, the total number of data bits which must be transmitted is reduced, while a satisfactory picture and sound are still provided.
With the increased use of digital-audio and video content for television, computer graphics, compact discs, digital cameras, and the like, it would be desirable to provide hidden data in digital audio and video signals in a number of situations. For example, it may be desirable to enable data ancillary to the audio or video data to be carried to provide a copy management scheme which precludes the audio or video signal from being copied without proper authorization, or otherwise controls the use of the audio or video program or other information (e.g., multimedia) associated with the video signal. Additionally, information identifying the content of the audio or video signal, such as the name, source, sponsor and/or performers of an audio or video program, and polling information for market research or commercial verification might also be hidden. The scheme should be compatible with various storage media, including optical storage media such as the Digital Video Disc (DVD), Compact Disc (CD) including CD-ROM, and magnetic storage media such as Video Cassette Recorder (VCR) tape.
Further, the type of ancillary information carried by the audio or video signal should be virtually unlimited. For example, it would be advantageous to allow data that is completely unrelated to the audio or video signal to be carried (e.g., transmitting stock price data or "electronic coupons" with a movie). Moreover, the scheme used to hide data in the audio or video signal should be able to hide either a modulated carrier, an unmodulated carrier (e.g., pilot), or a combination of both.
It would further be desirable for the auxiliary information to be substantially imperceptibly transported concurrently with a primary audio or video signal. In one such technique, the bandwidth of the audio or video channel remains as is, and additional information is packed with the audio or video information such that it can be retrieved without significantly degrading the quality of the primary audio or video signal.
The scheme should be compatible with audio or video decoders which do not have the capability to retrieve the auxiliary information as well as with special decoders which can retrieve the auxiliary information. Additionally, video decoders may have the capability to subtract the supplemental information from the video data prior to transforming the video data to the pixel domain in order to recover the video data with essentially no degradation in quality.
The scheme should be compatible with existing digital video data communication schemes, where a video signal is digitized, spatially transformed, compressed, packed in a predefined frame format, and then transmitted in a bitstream. The scheme should thus allow hidden data to be embedded in the compressed transform samples without requiring full decompression of the signal.
The scheme should provide a method for hiding auxiliary data in a digital audio or video data stream which is communicated over existing communication paths as a pre-existing bitstream, without substantially altering the quality of the audio or video signal, or interfering with other data in the bitstream.
In particular, the scheme should allow the provision of auxiliary data in respective subbands (e.g., spatial frequencies) of a digital compressed video data stream, or in respective subbands of a digital compressed audio data stream. Similarly, the scheme should allow recovery of the auxiliary data from the compressed digital audio or video signal. Finally, the scheme should provide spectral shaping of the auxiliary signal to enhance its concealment.
The present invention relates to a method and apparatus for transporting and recovering information hidden in a digital audio or video signal having the aforementioned and other advantages.